Time Weighted Average (TWA8)

The 'TWA8' is the Noise Level (dBA) which, if constant over
8 hours, would result in the same Noise Dose as the one calculated using the
actual exposure time.

Example: If a worker's Noise Dose from a 10 hour shift is
400%, what constant Noise Level (in dBA) over 8 hours would produce the same
Dose? The answer is on the Noise Dose Table. Find "400" in the the
"8 hour" Exposure Time column. Follow the row (left) to the Noise
Level, which, in this case, is 100 dBA. The TWA8 value for any Noise Dose
equaling 400% is 100 dBA .

Why use TWA8? Why not just use Noise Dose?

Because the "TWA8" (which is expressed in dBA, rather than a
percentage) is easier to use when applied to noise controls.

Example: If one knows that the noise level in an area is kept below 90 dBA,
for the full 8 hour work shift, then the 90dBA PEL has not been exceeded. Trying
to think of this in terms of noise dose percentages is awkward.